I would like to copy the reaction scheme similar to the image provided. This is what I have attempted so far, but I am new to chemfig so I'm not sure what I'm doing. Please provide your expertise. Thanks.
\documentclass[tikz]{standalone}
\usepackage{chemfig,siunitx}
\usetikzlibrary{calc,arrows,shapes,positioning}
\DeclareSIUnit{\molar}{M}
\pgfplotsset{compat=newest}
\pagestyle{empty}
\setcompoundsep{7em} % (not exactly) the length of the arrows
\begin{document}
\schemestart
H \arrow(--HH){<=>[$K_{HH}$]}[90] HH \+ G \arrow(--GG){<=>[$K_{GG}$]}[-90]GG
\arrow{<=>[$K_{1}$]} HG
\arrow{<=>[$K_2$]} $HG_2$
\schemestop
\end{document}